It is well-known in the glass art to coat glass sheets with metallic and/or dielectric materials to impart enhanced solar and optical properties to the glass sheets. For example, it is known to place multiple layers of metals and dielectrics onto glass to produce electrically conductive coatings which are transparent to visible light and yet highly reflective to infrared radiation. It is also known to deposit conductive metal oxides onto glass, such as fluorine-doped tin oxide, which are also highly reflective to infrared radiation.
Tungsten oxide is a transparent semi-conductor, which when doped with fluorine shows an appreciable increase in its infrared absorption and reflection. Hence, fluorine-doped tungsten oxide films are potential candidates as coatings on glass for solar control applications. By reflecting and/or absorbing energy in the infrared, these coatings when applied to glass reduce the energy influx into a building or motor vehicle by as much as 50 percent, thereby reducing the need for air-conditioning systems.
Many techniques for depositing metal and dielectric coatings onto glass are well-known. Examples of conventional deposition techniques include liquid or powder spray pyrolysis, wherein liquids or solid particles containing film forming reactants are sprayed onto the surface of a hot glass ribbon being produced by the well-known float glass process. A more convenient method for depositing coatings onto glass is by way of chemical vapor deposition, wherein vaporized film-forming precursors are reacted at or near the surface of a hot glass ribbon to form the metal or dielectric film thereon. Chemical vapor deposition does not suffer from the problems associated with either liquid or powder spray pyrolysis techniques. That is, the liquid spray pyrolysis technique substantially cools the hot glass ribbon, while the powder spray pyrolysis technique requires a complex, delicate powder handling and delivery system.
One method for producing fluorine-doped tungsten oxide films on glass is disclosed in U.S. application Ser. No. 806,177, now U.S. Pat. No. 5,286,520 filed Dec. 13, 1991 to Proscia and commonly assigned with the present invention. It discloses a process for forming a coating on glass by reacting at the surface of the glass at elevated temperatures, tungsten hexafluoride, an oxygen containing compound, and a fluorine-containing compound such as difluoroethane. The disadvantage of that process is that it produces corrosive hydrogen fluoride as a by-product. It would be desirable to prepare a fluorine-doped tungsten oxide film by a process which overcomes the above deficiency.